1. Field of the Invention
The present invention relates to timing recovery in a data communications system and, in particular, to synchronous detection of a Frequency Shift Keying (FSK) signal in a modem receiver.
2. Discussion of the Prior Art
The basic function of any communications system is to transmit information over a communication channel from an information source to a destination as fast and as accurately as possible.
There are two general types of information sources. Analog sources, such as a telephone microphone, generate a continuous signal. Digital sources, such as a digital data processing system, generate a signal that consists of a sequence of pulses.
Communications channels that are designed to transmit analog signal (e.g., the telephone network) have characteristics which make it difficult for them to transmit digital signals. To permit the transmission of digital pulse streams over an analog channel, it is necessary to utilize the digital data pulses to modulate a carrier waveform that is compatible with the analog transmission channel.
The equipment that performs the required modulation is generally referred to as a "MODEM". The term "MODEM" is an acronym for MOdulator-DEModulator, since one piece of equipment typically includes the capability not only to modulate transmitted signals, but also to demodulate received signals to recover the digital data from the modulated analog carrier waveform.
While passing through the transmission channel, the modulated carrier waveform suffers from distortion introduced both by the system itself and by noise contamination. Thus, one of the tasks of the modem's demodulating receiver function is to filter the signal received from the transmission channel to improve the signal-to-noise ratio. The modem receiver also recovers timing information from the received signal to provide sampling points for recovering the digital data. The modem receiver may also condition the data in other ways to make it suitable for additional processing.
In a conventional modem, the signal filtering, timing recovery and conditioning tasks are performed by three functional units: analog-to-digital conversion circuitry ("analog front end") that converts the received modulated carrier waveform to a digitized replica, a digital signal processor (DSP) that retrieves the digital data from the digitized replica using a recovered timing signal, and a control function for controlling both the analog front end and the DSP. The DSP recovers the data by implementing a signal conditioning and data recovery algorithm that is specific to the type of data being received.
For example, the DSP function in a facsimile (fax) machine modem implements a special purpose algorithm that can only be used for recovering digital fax data. In the case of a fax system, the data to be recovered is a digital bit map that corresponds to the transmitted hard copy image and which has been compressed to facilitate efficient transmission. The algorithm implemented by the digital signal processor function of the receiving fax machine's modem is a dedicated "fax" algorithm that has been designed specifically for accurately recovering the compressed bit map. It cannot recover digital data in a format other than a compressed bit map, e.g. voice mail data or data modem applications. A different digital signal processor implementing a different dedicated "voice mail" or "data modem" algorithm is needed for each of these other applications.
As shown in FIG. 1, a conventional fax machine architecture may be partitioned into two major functional blocks: a special purpose fax modem block 1 of the type described above for recovering a compressed bit map from a modulated carrier waveform and a general purpose processor block 2 for performing those tasks required to convert the compressed bit map to a corresponding hard copy image.
Data transmission systems that operate at low transmission rates, i.e. 1200 baud or less, typically utilize a modulation technique known as Frequency Shift Keying (FSK). According to the FSK technique, the two binary states are represented by two different frequencies and are detected using two frequency tuned sections, one tuned to each of the two frequencies. The demodulated signals are then integrated over the duration of a bit period and a binary decision is made based on the result.
A common disadvantage of conventional analog FSK demodulator circuits is that they are sensitive to circuit parameter variations and are not suitable for large scale integration.
Conventional digital FSK demodulator signal processing requires A/D converters, high power consumption and very high system clocking frequencies which results in major expenditures for RFI shielding.
Moreover, conventional FSK modems are asynchronous modems that are used to transfer asynchronous data over the communication link, usually with start and stop delimiters. When synchronous data are to be transmitted through such a modem, a bit synchronizer (FIG. 2) must be added to the modem in order to synchronize on the exact timing. Combining the modem receiver and bit synchronization functions in cascade (i.e. asynchronous detection and timing) is inefficient and results in reduced performance.